In heat exchange systems or equipment such as air conditioning and refrigeration circuits, the use of a coaxial tube in tube heat exchanger is a known expedient and such devices are easily purchaseable on the open market such as the devices manufactured and sold by Edwards Engineering Corporation of Pompton Plains, New Jersey.
Such coaxial tube in tube heat exchangers include generally an elongated continuous outer tube of predetermined length, an elongated continuous inner tube having a length slightly greater than the outer tube lying coaxially in the longitudinal line of the outer tube and extending out of the opposite ends thereof, said outer tube and inner tube defining an annular fluid flow passage therebetween, and a terminal inlet connection for one end of said annular fluid flow passage and a terminal outlet connection spaced from the inlet connection for the opposite end of said annular fluid flow passage. These elements are brazed together generally at opposite ends of the outer tube and then the coaxial units are rolled or turned into various desired shapes such as spirals, toroids, helixes, trombones, etc. for the particular application to which they will be put. In the operation of these devices cooling fluid passes through the inner tube and gas to be condensed is passed in counter current flow through the annular fluid flow passage, the condensate flowing from the terminal outlet connection.
In order to increase the efficiency of these condensers, the inner tube for the greater length thereof is spirally wound with radially extending fins which are solder bonded to the outer surface of the inner tubes to provide metallic bonding of the fins thereto. Further the pitch of the spirally wound fins is such that a predetermined gap is established between each successive spiral to optimize both the bending and forming of the selected shape of the condenser and for insuring proper drainage of condensate from the annular fluid flow passage. My U.S. Pat. Nos. 2,661,525, 2,635,571 and 2,529,945 show and describe this type of finned tubing.
In order to establish and insure safe operating conditions various commercial regulations have been established for pressure vessels and a product to be commercially viable must be tested and certified by the Underwriters Laboratory which is the commercial agency qualified to give these certifications. Such approval and certification is granted based on various standardized hydraulic tests to which such pressure vessels are subjected. The tests which are applied to coaxial tube in tube heat exchangers require the vessel walls to withstand pressures in excess of five times the pressures normally met during ordinary operating conditions. For these heat exchangers the highest operating pressure is about 400 P.S.I.G. Accordingly, the walls of the tubular members and joints in these heat exchangers must be able to withstand, under test, hydraulic pressures in excess of 2000 P.S.I.G.
Heretofore when such tests have been applied to the known prior art type of coaxial tube in tube heat exchangers, due to the collapse of the inner tube under these pressures, ruptures have resulted more particularly at the brazed joints which hold the outer tube, inner tube and terminal connectors in assembled position.
In order to meet the pressures such heat exchangers must withstand particularly in the standardized hydraulic tests, it has been accepted prior art practice to increase the wall thickness of the copper or cupronickel inner tube being utilized in such coaxial tube in tube heat exchangers. Due to the increase in the cost of the materials for manufacturing the tubes from which such heat exchangers are manufactured this practice obviously increases the overall cost of such coaxial tube in tube heat exchangers.
In the present invention this problem is overcome by providing a support insert of sufficient length in the inner tube in operative alignment with the associated brazed connection for connecting the outer tube, the inner tube and the terminal fitting at a given end of the coaxial tube in tube heat exchanger.
This construction not only overcomes the problem but provides the additional advantage of permitting the wall thickness of the inner tube to be materially reduced which factor produces a substantial reduction in the overall manufacturing costs for such coaxial tube in tube heat exchangers.